The use of portable battery operated devices that employ very complex high performance electronic circuitry has increased dramatically over recent years with the widespread use of cellular telephones and laptop computers, among other devices. In addition, for precision coders/decoders (CODECS), the conversion accuracy of signals from analog to digital and back again is directly dependent on the stability of the reference voltage. For proper and reliable operation, such devices require a reference or bandgap voltage, V.sub.BG, typically of about 1.25 volts, that is stable and immune to temperature variations, power supply variations and noise.
It is also desirable for the reference voltage, V.sub.BG, to be driven by a power source, V.sub.DD, that can retain power for long periods of time before recharging is required. To meet this requirement, a number of batteries or a single large battery is generally needed, thereby increasing the size and weight of the overall device and making the device less desirable or suitable for portable use. However, if the voltage of the power source, V.sub.DD, can be minimized, the number and size of the batteries required may also be reduced.
Typically, a circuit known as a bandgap voltage reference generator is used to provide the required stable reference or bandgap voltage, V.sub.BG. A CMOS bandgap voltage reference generator with a high power supply rejection ratio (PSRR)--the ratio of the change in the power source, V.sub.DD, to a change in bandgap voltage, V.sub.BG --which is useful, for example, in analog integrated circuits is disclosed in U.S. Pat. No. 4,849,684. In that device, a magnified current derived from a thermal voltage reference produces a voltage drop across a resistor. The resistor is coupled to a bipolar transistor which is part of the thermal voltage reference. The bandgap voltage is the sum of the voltage across the resistor and the voltage across the bipolar transistor. The bandgap portion of the disclosed circuit itself has a PSRR of only about 30-40 decibels. A differential amplifier senses the voltages at the control current input and the output of a current mirror in the thermal voltage reference portion of the bandgap voltage reference and adjusts the power supply voltage to the thermal voltage reference until the sensed voltages are substantially the same. The differential amplifier enhances the PSRR of the circuit to about 100 decibels.
Although the bandgap voltage reference generator disclosed in U.S. Pat. No. 4,849,684 is reliable, functional and useful for many applications, its circuitry requires a power source, V.sub.DD, of at least about 4 volts to produce a reference or bandgap voltage, V.sub.BG, of about 1.25 volts. This minimum voltage level of the power source, V.sub.DD, is due to the fact that the regulator transistors (FETs 22 and 23) produce a threshold voltage drop of about 3 volts. Consequently, the voltage source, V.sub.DD, must be in excess of at least about 4 volts to produce an output bandgap voltage, V.sub.BG, of about 1.25 volts.